IR black body calibration sources ("calibration ovens")are typically made such that the heated black body is an Aluminum ingot placed behind a matte painted sheet of Aluminum with a hole in it slightly smaller than the ingot. This keep fringe errors low near the edges of the ingot, and cuts down on ambient convection currents wisping past your target when you are trying to calibrate an IR device.
The ingot is called "the target" and the "target size" equals the size of the hole or "port" in the Aluminum sheet.
He already stated what he wanted his targeting capability to be. 3mm.
My test solves two problems at once. Targeting capacity, and amplitude when on target at a given temperature.